In a typical electrical machine control algorithm, at drive power-up, the initial electrical rotor position is not known and a dedicated identification algorithm has to be used. The initial electrical rotor position is an important control parameter that allows a controlled operation at zero speed without reverse rotor rotation or over-speed fault.
The standstill solutions found in literature are mainly divided into two categories: (1) transient excitation tests, also known as pulse tests, which use a number of consecutive voltage pulses applied, in an open-loop manner, to consecutive phases of the electrical machine. The pulses are applied for a given amount of time. At the end of each voltage pulse, phase current samples are taken. The phase current samples are used to evaluate the electrical rotor position and the permanent magnet (PM) flux polarity. (2) High-frequency signal injection methods, which are more advanced and have better accuracy relative to the pulse methods. However, they require additional signal processing techniques and waveform analysis.
The two main ways of implementation of a high-frequency signal injection methods use either rotating injection or pulsating injection techniques. In the case of the rotating injection method, a high-frequency sinusoidal voltage vector rotating at constant speed is applied to the electrical machine. The measured carrier current is processed and the electrical rotor position and magnet polarity information are transmitted as a phase modulation between the injected rotating voltage and the measured rotating current response.
For the pulsating injection method, a randomly orientated high-frequency sinusoidal pulsating voltage vector is applied in the rotor reference frame of the electrical machine. The electrical rotor position and permanent magnet flux polarity information are extracted as an amplitude modulation of the estimated rotor reference frame carrier current response.
The solutions proposed in literature are commonly developed on particular electrical machines and lack a generalized approach. As a result, when applied on different electrical machines, these solutions require algorithm modifications, tuning of certain parameters of the algorithm or identifying various machine dependent parameters. This is a major drawback since this identification algorithm is applied during drive start-up when only basic information of the electrical machine is available: nominal current, nominal voltage and nominal frequency. Furthermore, some solutions proposed in the literature require the use of a rotor position feedback device. However, for some applications, such a device is too costly or has a negative influence on the reliability of the drive and therefore is omitted. As a consequence, some proposed methods found in literature cannot be applied to industrial drives.
In the case of the pulse tests (1), a brief problem statement is the duration of application of the voltage pulse. This is critical when standstill operation is mandatory. As a result, the pulse duration is typically determined a priori through experimental tests and then stored in the memory of the drive associated with the respective electrical machine.
In the case of the high-frequency injection methods (2), there is a need of prior tuning of some parameters used for the current regulation or for the position and permanent magnet flux polarity demodulation algorithm. Additionally, the high-frequency signal injection methods are sensible to the saliency ratio value, a machine specific parameter. The demodulation algorithm for the electrical rotor position can be unstable if it is not designed for a specific saliency ratio, which can be typically greater or lower than one, therefore a specific machine design. Furthermore, a machine with a saliency ratio close to one may lead to electrical position identification failure.
US20100264861 discloses a method for determining the position of the flux vector of an electric motor which is driven by a variable speed drive. The method is achieved without a speed or position sensor, and is based on the detection of an error in the estimated position of the flux vector by using a low-frequency current injection. The method includes injecting a first current vector into a first injection reference frame rotating at a first frequency relative to a reference frame synchronous with the rotation of the motor, and a second current vector into a second injection reference frame rotating at a second frequency opposite to the first frequency, a step of determining a first stator flux induced voltage delivered at the output of a first integrator module synchronous with the first reference frame and a second stator voltage delivered at the output of a second integrator module synchronous with the second reference frame, a step of regulating the position of the rotor flux vector by minimizing the error, between a real position and an estimated position of the rotor flux vector, the error being determined based on the second induced voltage.
According to US20100264861 the rotor has to be moved at least a few degrees back and forth because it requires the generation of a back electromotive force. For certain industrial application rotor movement is however not allowed during the parameter identification.